She said the world must brace itself for further deaths, although she does not expect to see a sudden leap in the number of people with severe or fatal infections.

Dr Chan added there was now “indisputable evidence” of a global pandemic caused by a new H1N1 virus.

Health Minister Edwina Hart last night told Assembly Members: “The reality is this doesn’t change anything with how we deal with this on the ground.

“We don’t want people panicking with the word pandemic.”

And health experts stressed that pandemic only refers to the spread of the swine flu virus and not its severity.

Dr Maureen Baker, honorary secretary of the Royal College of GPs (RCGP), said: “This does not mean that patients are in any more danger, or that the virus has become any more harmful. The majority of H1N1 cases will get a nasty dose of flu and recover within a week or two.”

There are still only two confirmed cases of swine flu in Wales – a 31-year-old man in Caerphilly and Dr Eleanor Mair Williams, 66, from Swansea – have both recovered after suffering mild symptoms after returning from the US.

But there have now been more than 28,000 cases, including 144 deaths in 74 countries worldwide. The number of cases in the UK last night rose to more than 850.

More than 20 schools across the UK have closed as a result of swine flu.

In a statement last night Mrs Hart said: “I know that it is natural for people to feel apprehensive about the WHO’s move, but I would like to reassure the public that we have been planning for this for some time and have robust procedures in place to deal with it effectively.

“There will be more cases in Wales as the infection spreads in the community and everyone must continue to be vigilant and try to reduce the spread of the virus.

“Our advice to the public remains the same, people should follow good respiratory and hand hygiene to help prevent the spread of influenza – always using a tissue to catch sneezes, throwing away used tissues where germs can linger and killing germs by regularly washing your hands, or cleaning them with a sanitising gel. In short, catch it, bin it, kill it.

“In particular, we know that we are in our summer now and need to be prepared for the possibility of a more severe second wave of the virus in the winter months.

“We will continue to monitor the situation closely in Wales with our National Public Health Service and NHS Wales and keep the public informed.

“We also continue to work with the other UK governments to plan and co-ordinate our response according to the UK pandemic plans and in the light of our local circumstances.”

Professor Steve Field, chairman of the RCGP, who worked on the UK’s pandemic flu plans, said: “This is a historic time because we have not had a flu pandemic in more than 40 years.

“I believe we have been at this point for a couple of weeks or so, so this declaration doesn’t come as a surprise.

“For the UK it doesn’t mean any real change to what we’ve been doing in the last few weeks.”

WHO said it considers the overall severity of the flu pandemic to be moderate as most people have recovered from infection without the need for hospitalisation or medical care.

And it said that overall, national levels of severe illness from influenza A(H1N1) appear similar to levels seen during seasonal influenza periods.

Hospitals and healthcare systems in most countries have so far been able to cope with the numbers of people needing help.

But in its assessment of the pandemic, it said it is concerned about patterns of serious cases and deaths that are occurring largely among young persons, including those who were previously healthy, had pre-existing medical conditions or were pregnant.

Dr Chan said about the pandemic declaration: “This sends an important message to countries that, irrespective of what stage the epidemic is in their country, they must maintain continuous vigilance. The virus is full of surprises.

“Countries that are not reporting cases must be on the watch for the arrival of the infection and start to prepare their people and healthcare systems to deal with the arrival of this new disease.

“In a situation like the UK’s – a country in transition – you can only do containment for some weeks and if we are seeing continuing spread of the infection in the community it is important that they move to mitigation messages, which they are doing.

“That is a smart way to preserve manpower resources in the system so you can focus on identifying patients, the early treatment of patients, making sure vulnerable groups get proper and timely treatment to prevent deaths.”

She added: “No previous pandemic has been detected so early or watched so closely in real time right at the beginning.”